Complex Formation with the Type B γ-Aminobutyric Acid Receptor Affects the Expression and Signal Transduction of the Extracellular Calcium-sensing Receptor: STUDIES WITH HEK-293 CELLS AND NEURONS

We co-immunoprecipitated the Ca2(+)- sensing receptor ( CaR) and type B gamma- aminobutyric acid receptor ( GABA-B-R) from human embryonic kidney ( HEK)- 293 cells expressing these receptors and from brain lysates where both receptors are present. CaRs extensively co-localized with the two subunits of the GABA- B-R ( R1 and R2) in HEK- 293 cell membranes and intracellular organelles. Coexpressing CaRs and GABA- B- R1s in HEK- 293 cells suppressed the total cellular and cell surface expression of CaRs and inhibited phospholipase C activation in response to high extracellular [ Ca2(+)] ([ Ca2(+)] (e)). In contrast, coexpressing CaRs and GABA- B- R2s enhanced CaR expression and signaling responses to raising [ Ca2(+)] (e). The latter effects of the GABA- B- R2 on the CaR were blunted by coexpressing the GABA- B- R1. Coexpressing the CaR with GABA- B- R1 or R2 enhanced the total cellular and cell surface expression of the GABA- B- R1 or R2, respectively. Studies with truncated CaRs indicated that the N- terminal extracellular domain of the CaR participated in the interaction of the CaR with the GABA- B- R1 and R2. In cultured mouse hippocampal neurons, CaRs co-localized with the GABA- B- R1 and R2. CaRs and GABA- B- R1s also co-immunoprecipitated from brain lysates. The expression of the CaR was increased in lysates from GABA- B- R1 knock-out mouse brains and in cultured hippocampal neurons with their GABA- B- R1 genes deleted in vitro. Thus, CaRs and GABA- B- R subunits can form heteromeric complexes in cells, and their interactions affect cell surface expression and signaling of CaR, which may contribute to extracellular Ca2(+)- dependent receptor activation in target tissues.